1. Field of the Invention
The present invention relates to the field of automotive transmissions, and more particularly, to a full support mainshaft and fifth gear design for the New Venture/MTM NV4500 transmissions.
2. Description of the Related Art
“Losing” fifth gear is a common complaint with the NV4500 transmissions, particularly when they are coupled to a diesel engine. Although the nut is often blamed, it is actually the mainshaft that fails. The problem is caused by inadequate support of the gear on the mainshaft.
The factory gear is fully splined on its inner surface. On the outside of the gear, approximately half the gear is covered with gear teeth. In the area below the gear teeth, only the tips of the inner gear splines are in contact with the smooth support area of the mainshaft. Thus, the gear is not adequately supported by the mainshaft, and the tips of the splines actually wear into the smooth support area of the mainshaft until the gear is no longer a press fit to the shaft. Once the gear is loose, it either works the mainshaft nut loose until it spins off, which allows the gear to move backward and out of alignment with the mating countershaft gear, or the gear spins on the shaft even though the nut stays on. In either case, the ability to use fifth gear is lost. Installing a new fifth gear and/or nut on a worn mainshaft is only a short-term repair, and the imbalance caused by the wobbling gear often results in transfer case damage. Welding the gear onto a worn mainshaft does not work, nor does LOCTITE®, a high-performance adhesive manufactured by Henkel Technologies. Fifth gear is no longer available until the transmission is repaired.
Both the original equipment manufacturer and the aftermarket have introduced various nut designs in an effort to remedy this problem and keep the gear on the shaft. Aftermarket manufacturers have also tried to correct this problem with two variations of mainshafts that have full-length splines for the fifth gear. The first variation of the aftermarket mainshaft was designed for a fairly light press fit between the gear and shaft. It also uses a spring washer and flat nut to hold the gear onto the shaft. The second variation of the aftermarket mainshaft uses a slip fit between the gear and shaft plus a three-piece thrust washer (consisting of a split washer and a retaining ring) between the mainshaft lock washer (a spring washer) and fifth gear. None of these “fixes” has been successful in preventing fifth gear failures.
FIGS. 1–14 illustrate the prior art. FIG. 1 is a perspective view of a new factory mainshaft 1 without the gear. This figure shows the splines 2, smooth support area 3, and threaded area 4 of the mainshaft. The gear fits over the splines 2 and the smooth support area 3.
FIG. 2 is a partial perspective view of a factory mainshaft 1 with the factory gear 5, spring washer 6 and flat nut 7. The washer 6 and nut 7 fit over the threaded area 4 of the shaft. In an early attempt to solve the problem of the gear working itself off of the shaft, the spring washer 6 was added between the gear 5 and the nut 7. The addition of the washer, however, did not solve the problem.
FIG. 3 is an exploded perspective view of the factory gear 5, spring washer 6 and flat nut 7 of FIG. 2. The nut 7 comprises notches 7a that allow the nut to be turned with a wrench. In the prior art, the nut typically has a beveled edge (not shown) facing the spring washer. The beveled edge has no functional purpose.
FIG. 4 is a perspective view of the factory gear 5 showing the full inner splines 8. As shown in this figure, the splines are triangular in shape with blunt (as opposed to pointed) tips.
FIG. 5 is a partial perspective view of a used factory mainshaft 1 showing the area where the gear 5 (not shown) wears into the shaft. As shown in this figure, the smooth support area 3 is no longer smooth but actually has subtle indentations that have been worn into it by the gear splines 8 (see FIG. 4). These indentations cause the gear to work loose. Once the gear is loose, it works the nut and washer loose, which leads to fifth gear failure. Note that the diameter of the support surface 3 on the factory mainshaft 1 is smaller in diameter than the splined area 2. That difference in diameter is a key factor in the problem addressed by the present invention.
FIG. 6 is a perspective view of the first aftermarket mainshaft variation without the gear. This variation is a press fit aftermarket shaft 9. As shown in this figure, there is no smooth support surface 3 (see FIGS. 1 and 5). Instead, the splined area 10 has been extended so that the entire area below the gear is splined. The shaft still contains a threaded area 4, which further comprises a keyslot 11. The purpose of the keyslot 11 is illustrated in FIGS. 7 and 8.
FIG. 7 is a partial perspective view of the aftermarket mainshaft 9 shown in FIG. 6 with the factory gear 5, spring washer 6 and lipped nut 12. The washer 6 and nut 12 fit over the threaded area 4 of the shaft. The lipped nut 12 represents another attempt to solve the problem of the gear working loose. With this type of nut, part of the lip 12a is bent and pressed down into the keyslot 11. The thought was that the lip 12a in the keyslot 11 would prevent the nut 12 from coming loose, even if the gear 5 came loose. This nut design did not work either. FIG. 8 is an exploded perspective view of the factory gear 5, spring washer 6, and lipped nut 12.
FIG. 9 is a perspective view of the second aftermarket mainshaft variation without the gear. This variation is a slip fit aftermarket mainshaft 13 showing a groove 14 for a split washer (shown in FIG. 10). The groove is located between the splined area 10 and the threaded area 4. As with the press fit aftermarket shaft, the splined area 10 is extended so that the entire area below the gear is splined, and the threaded area 4 includes a keyslot 11.
FIG. 10 is a partial perspective view of the slip fit aftermarket mainshaft 13 shown in FIG. 9 with the gear 5, split washer (not shown) and retaining ring 15, spring washer 6, and lipped nut 12. the split washer and retaining ring fit within the groove 14 shown in FIG. 9. The spring washer 6 and nut 12 fit over the threaded area 4 of the shaft. As in FIG. 7, the lip 12a of the nut is pressed down into the keyslot. Because the configuration shown in FIG. 7 was not effective in keeping the gear on the shaft, the split washer (not shown) and retaining ring 15 were added. FIG. 11 is an exploded perspective view of the gear 5, split washer 16 and retaining ring 15, spring washer 6, and lipped nut 12.
None of the designs described above was successful in preventing fifth gear failure, and alternate nut designs were developed with the hope that they would serve to keep the gear on the shaft. FIG. 12 is a perspective view of a clamp-style nut 17. This nut has horizontal screws or clamps 18 that were intended to hold the nut more tightly on the threaded area 4 of the mainshaft. FIG. 13 is a perspective view of a nut with set screws 13. This nut comprises vertical screws 20 that are screwed down against the threads of the threaded area 4 on the mainshaft. Like the other designs described above, neither of these nut designs was effective in solving the problem.
With both the factory shaft and the aftermarket shaft, the gear is not fully supported by the area underneath it on the shaft. With the factory shaft, the smooth support area is smaller in diameter than the splined area so that only the tips of the inner gear splines are in contact with the smooth support area. With the aftermarket shaft, although the area beneath the gear is fully splined, the inner splines of the gear are not fully congruent with the splines on the shaft. This point is illustrated in FIG. 14, which is a partial section view of the gear 5 on an aftermarket mainshaft. The splined area of the mainshaft 10 is shown in relation to the inner gear splines 8. As shown in this figure, there are gaps between the inner gear splines 8 and the splines 10 on the mainshaft. These gaps lead to instability of the gear on the shaft and, ultimately, the loosening of the gear on the shaft.
It is an object of the present invention to solve the problem described above by providing a mainshaft and fifth gear design that affords more adequate support for the gear on the shaft and that does not result in fifth gear failure. It is a further object of the present invention to provide an embodiment that constitutes a retrofit of an existing factory mainshaft and an embodiment that is an entirely new mainshaft design.